Understanding the feeding role of tiger sharks

Tiger sharks are one of the most successful large predators in the world’s oceans, but studying what they eat has been a challenge for researchers. Historically diet is studied through examining stomach contents, but scientists at the Australian Institute of Marine Science (AIMS) and collaborators are leading the way in understanding more about the feeding habits of sharks from their skin tissue. This allows us to learn about shark diet based on a quick non-lethal approach.

In a recent study, AIMS marine biologists Dr Luciana Ferreira, Dr Michelle Thums, Dr Mark Meekan and co-authors from a number of Australian universities, revealed their findings after examining the tissue samples of 273 tiger sharks from Western Australia to New South Wales and the Great Barrier Reef.

Samples of blood and muscle tissue of tiger sharks showed information on the prey, position of individual sharks in the food chain, and even what type of habitat (coastal or offshore, seabed or open water) the animal had been feeding in.

Dr Ferreira said tiger sharks are large mobile animals and to ensure sustainable and resilient populations, we need better data on their feeding and behaviours.

“In terms of predators, if we can understand the shark’s motivations and how they are using habitats, we can also understand their function within these habitats,” she said.

Difficulties involved in the direct observation of feeding behaviour of marine megafauna such as tiger sharks, has led to the use of alternative techniques to provide insights into the process of their foraging. One of the most common of these is the analysis of stable isotopes of carbon and nitrogen in their tissues, which can provide information on diet, feeding position in the food chain and interactions among different species, and their migratory movements.

“Our analysis of stable isotopes shows that the functional role of tiger sharks in food-webs varied among different marine habitats we sampled along the tropical and temperate coasts of Australia,” Dr Ferreira said.

“Tiger sharks in Shark Bay and Ningaloo Reef in Western Australia, and on the Great Barrier Reef had long-term diets based in seagrass and reef-associated food webs. In these habitats they are focussed on turtles and dugong as prey.

“In contrast, when sharks were sampled in more temperate habitats such the waters off New South Wales and southern Queensland, the composition of their tissues reflected a diet based on more pelagic [ocean-going] food webs, and they were focused on large fish.

“Tiger sharks occupied roles at the top of food webs at Shark Bay in Western Australia and on the Great Barrier Reef, but not at Ningaloo Reef or off the coast of NSW.

“This means the local environment and prey community appear to be the most important determinants of the diet of tiger sharks.” Dr Ferreira said the research confirmed the role of tiger sharks in Australian coastal ecosystems as opportunistic, flexible predators.